A nappy cover provides a waterproof layer over the top of a fitted nappy, terry nappy, pre-folds or inserts. Without these your nappies are just going to leak all over! A nappy cover might close with hook & loop or snaps at the waist, and may be sized or one-size fits all. A two part nappy system, to include a waterproof nappy cover, is the preferred choice for overnight use for many families.
For more information on the different types of modern cloth nappies, you can find our handy guides here